In a typical cellular system, also referred to as a wireless communications network, wireless terminals, also known as mobile stations and/or user equipment units communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket, hand-held, computer-comprised, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called “NodeB” and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. Long Term Evaluation (LTE) together with Evolved Packet Core (EPC) is the newest addition to the 3GPP family.
A multi-carrier system (or interchangeably called as the carrier aggregation (CA)) allows a user equipment to simultaneously receive and/or transmit data over more than one carrier frequency. Each carrier frequency is often referred to as a component carrier (CC) or simply a serving cell in the serving sector, more specifically a primary serving cell or secondary serving cell. The multi-carrier concept is used in both HSPA and LTE. Carrier aggregation is supported for both contiguous and non-contiguous component carriers, and component carriers originating from the same eNodeB need not to provide the same coverage. Furthermore, carriers may also belong to different RATs. Below definitions are provided for various cells in a CA network.
Serving Cell: for a user equipment in RRC_CONNECTED not configured with CA there may be only one serving cell comprising the primary cell. For a user equipment in RRC_CONNECTED configured with CA, the term ‘serving cells’ is used to denote the set of one or more cells comprising of the primary cell and all secondary cells.
Primary Cell (PCell): the cell, operating on the primary frequency, in which the user equipment either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or the cell indicated as the primary cell in the handover procedure.
Secondary Cell (SCell): a cell, operating on a secondary frequency, which may be configured once an RRC connection is established and which may be used to provide additional radio resources.
In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC) while in the uplink it is the Uplink Primary Component Carrier (UL PCC). Depending on user equipment capabilities, Secondary Cells (SCells) may be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to aSCell is a Downlink Secondary Component Carrier (DL SCC) while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
In CA the base station, e.g., eNode B, in LTE may deactivate one or more secondary cells on the corresponding secondary carriers. The deactivation is done by the eNB using lower layer signaling, e.g., over PDCCH in LTE, using a short command such as ON/OFF, e.g., using 1 bit for each SCell. The activation/deactivation command is sent to the user equipment via the PCell. Typically, the deactivation is done when there is no data to transmit on the SCell(s). The activation/deactivation may be done independently on uplink and downlink SCell. The purpose of the deactivation is thus to enable user equipment battery saving. The deactivated SCell(s) may be activated also by the same lower layer signaling.
Herein, a cell change is referred to as changing the cell to which the user equipment is associated to. The cell change may further refer to a serving cell change, e.g., at handover in a non-CA system or when the user equipment is not configured with any SCell, a serving cell set change, e.g., in a CA system adding/removing/modifying an SCell, or a PCell change, e.g., in a CA system changing the current PCell being cell with the first cell identity to another cell with the second cell identity.
A cell change may occur during a handover (intra-frequency, inter-frequency or inter-RAT), a PCell change on the same PCC (in a CA system), or a carrier switching (changing the current PCC to another frequency carrier, which implies also PCell change). A cell change may be due to e.g. mobility, load balancing, energy saving, carrier activation/deactivation, etc.